How?
Present the Museum Day admission card to receive free general admission at participating Museum Day locations. The admission card is available in the September 2008 issue of Smithsonian magazine or downloadable via the Smithsonian site. You will need to select a museum from the venue page list that you want FREE admission to on September 27th when you apply for the card.

Note: Smithsonian is promoting membership, but you don’t need to be a member or subscribe to the magazine to participate!